This invention relates to improvements in hinged containers. In particular, it relates to a thermoformed container having uniformly thick walls joined by a living or "live" hinge structure.
Containers for food or other goods are currently manufactured by thermoforming polystyrene sheet. Typical clear thermoplastic containers are disclosed in U.S. Pat. No. 5,046,659 (Warburton), incorporated herein by reference.
It is also desirable to form a closed stackable container for food or other articles comprising a lid and a mating base each having a fluted pattern comprising a uniformly spaced series of stiffening wall flutes extending around the periphery of the container. The top and bottom container wall flutes in both of the flute patterns may be shaped to interlock with each other in stacking relationship.
The prior art thermoplastic containers are formed with a semi-rigid, resilient hinge joining the lid and base members and sufficiently flexible to permit bending 180.degree. during closure. The resiliency of a living hinge tends to hold the container in the open position unless closed and latched. This type of hinge is substantially the same thickness as the container (i.e. - about 0.4 mm or 0.015 inch). The relatively thick hinge tends to warp the side walls of the base and lid in the vicinity of the hinge, causing a gap in the container seal after closure. This is due to the relative greater stiffness of the hinge portion. While the hinge can be weakened by scoring or creasing the hinge line, this is not an acceptable technique for maintaining a living hinge and may not avoid gapping between the base and lid. Also, the side wall distortion and seal gap problem may be avoided by substituting a latch for the hinge; but, this loses the advantage of a one-piece, integrally formed article and increases the complexity of the closure.
It has been found that the closure warp problem can be overcome by providing stiffening to the adjacent walls by forming a spaced series of channels along a top shoulder of the base adjacent to the hinge, with each of the channels extending from the shoulder top to a sealing flange portion, thereby stiffening the base wall in the vicinity of the hinge. A mating series of protruding lugs is provided in the lid member opposite the channels adapted for closing into the channels and providing the same stiffening effect as the base.